A wavelength selective switch can receive an input light having any wavelength or provide an output light having any wavelength at any port, and it is an important optical device in a reconfigurable optical add-drop multiplexer (ROADM) system. In order to support higher modulation rate, more network channels and higher network flexibility, bandwidth-adjustable wavelength selective switches are attracting more and more attention. In order to realize adjustable bandwidth, many optical device manufacturers begin to develop wavelength selective switches based on liquid crystal technology.
The wavelength selective switches based on liquid crystal technology mainly includes the following several implementation forms:
LCOS (Liquid Crystal On Silicon) solution-based wavelength selective switches, which are adopted by Finisar and Santec, see U.S. Patent Application Publications No. US20060067611A1 and No. US20130128215A1. In this solution, voltages are applied to make the phases of the pixel units in corresponding region vary periodically, thereby generating diffraction property of a grating, and a deflection angle of the light can be controlled by adjusting the phase period. The optical path structure of this solution is simple, for example, only one LCOS chip is needed to realize multi-port output. Its disadvantages are that the driving circuit is complicated and that the LCOS chip is difficult to be manufactured. Therefore, this chip has very limited acquisition channel and very high cost.
Liquid crystal cell array+birefringence wedge solution-based wavelength selective switches, which are adopted by CoAdna, see U.S. Patent Application Publication No. US20080087378A1. Each layer of liquid crystal+birefringence wedge can control two optical path states, and thus N layers can obtain 2N optical path states, i.e., 2N output ports. As the number of output ports increases, the number of layers of liquid crystal also increases, which will increase packaging difficulty of the liquid crystal portion and deteriorate the bandwidth of the optical path.
Liquid crystal cell array+liquid crystal polarization grating solution-based wavelength selective switches, see Chinese Patent Application Publication No. CN103703405A filed by Huawei and published on Apr. 2, 2014. The photo-alignment layer of the liquid crystal polarization grating is exposed by coherent light beams, so that the incident light having different polarization states can be diffracted with different diffraction orders. This solution can realize up to 3N port outputs, but it also requires stacking of multiple layers of liquid crystal cells.